In commercial aircraft operations, an aircraft is conventionally towed by means of a tractor equipped with a towbar that is removably coupled to nose gear of the aircraft. Ground crews operate the tractor and towbar to move the aircraft during an aircraft departure operation, for example, to push the aircraft away from the gate into a position on the tarmac at which the towbar is decoupled from the nose gear, the tractor pulls clear and the aircraft is free to taxi under its own power.
Conventional towbars include wheels that support the towbar body above the ground and which allow the towbar to be maneuvered around by the tractor when not coupled to nose gear. When the towbar is coupled to nose gear, the wheels must be retracted upwardly free from the ground, thereby allowing the towbar to be held suspended between the tractor and the nose gear. When the aircraft has been moved into a desired position, the towbar must be decoupled, but not before the towbar wheels have been deployed to the ground in the support position.
Conventional towbars utilize a hydraulic hand pump and cylinder in a closed hydraulic system to “pump” from a retracted position down to the support position.
Towbars differ by manufacturer and aircraft type but are generally similar in design. Depending on type, each towbar typically weighs approximately 400 to 800 pounds. One end has an eyelet type of socket that matches up with a “pintle” hitch receiver that is standard on all push tugs. The other end of the towbar has a unique, for each airplane type, connector (“head”) that attaches the towbar to the nose-wheel of that aircraft. Over the years there has been a lot of engineering and development on this head to make it fast to connect and disconnect to the airplane. This head has a safety feature built in that has the ability to shear or break or in various fashions release potential side-loads to the towbar without completely disconnecting from the airplane. The steering systems on larger aircraft are hydraulically powered and this hydraulic powering is disabled during the push back operation but, if the pilot mistakenly connects the hydraulic power or the bypassing of it fails, the towbar can be immediately susceptible to an action that can be unsafe to men and equipment. Usually this is done with a “shear” bolt installed in the head of the towbar that is designed to shear before the side load gets too great. The push operation is then stopped, the reason for the hydraulic powering is found and corrected and the shear bolt is replaced and the push recommences.
As mentioned, on current towbars the wheels are “pumped” down with a hand pump and brought up by opening a valve at the pump that allows the hydraulic fluid to return to the pump's reservoir and they are brought up with large return springs. One might argue that the whole design could be reversed such that the springs could be used to bring the wheels down instead of up (and use the pump to move the wheels up), however this would be problematic in practice, as the springs would only lower the wheels until they contacted the ground and there would be no way to “fine tune” a lifting force to elevate the towbar sufficiently as needed to decouple it from the nose gear.
Typically, the push operation requires four people. One person drives the tug, one person walks the airplane back and wears a headset that keeps him in communication with the pilot and is the person in charge of the push, and two people who walk below the wingtips confirming clearance for the airplane as it is being pushed. The team pushes the airplane back far enough to allow the captain to turn out of the ramp area once released by a controller of the ramp area.
Conventionally, while the tug is pushing the plane via the towbar, the towbar has to be free from the ground. But once stopped, to disconnect the airplane, the end of the towbar nearer the airplane must have a set of wheels lowered to pick up the weight and allow the tug to pull the towbar away from the airplane. The current system uses a closed hydraulic system. There is a simple hydraulic pump mounted atop the towbar that connects to a cylinder via a hydraulic line that is hand pumped to cause a set of wheels to extend and pick up the weight for that end of the towbar. Once the wheels are pumped down and the head end of the towbar is aligned with the nose wheel connect point of the towbar, the weight is counter balanced, and the head can be disconnected from the nose wheel and the tug driver is given the command from the ramp person to pull away.
The time that it takes to pump the wheels down on a conventional towbar takes away from efficient operation, The captain has already started the engine during the push operation so as to be ready to move under ship power when the towbar is completely disconnected and clear of the airplane, The engine is running while this hand pumping goes on, so the aircraft is burning fuel while waiting for the ground crew to finish pumping the wheels down, The pumping operation requires several seconds, yet these seconds are costly when, for example, aircraft engines are consuming 600 pounds of fuel per hour, in a fleet running thousands of departures a day.